Light sensitive device



Oct. 18, 1966 F. H. NlcoLL LIGHT SENSITIVE DEVICE Filed March l, 1962 United States Patent O 3,280,357 LIGHT SENSITIVE DEVICE Frederick H. Nicoll, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 1, 1962, Ser. No. 176,712 Claims. '(Cl. 313-65) This invention relates to light sensitive devices. In particular this invention relates to light sensitive devices utilizing a photoconductor as the light sensi-tive element.

In the prior art there are many types of light sensitive image devices, e.g. pickup tubes and image tubes. Some of these imaging devices utilize photoemissive materials as the light sensitive element. When one utilizes the photoemissive materials as a light sensitive element, a limited value, e.g. -5 amps per 4square centimeter, is about the limit of the electron beam current which can be obtained from the known photoemissive materials. In many imaging devices it is desirable to have larger beam currents per unit of area than that which is obtainable with the use of conventional photoemissive materials.

It is therefore an object of this invention to provide a new and improved light sensitive device.

A further object of this invention is to provide a new and novel light sensitive device characterized in its use of a photoconductor as a light sensitive element to oontrol a copious electron emission.

These and other objects are accomplished in accordance with this invention by providing a photo-cathode in which a cold electron emitter, e.g. magnesium oxide, is used as an electron emitter with the emission therefrom controlled by means of a photoconductor material.

The invention will be more clearly understood by reference to the accompanying single sheet of drawings Wherein:

FIG. 1 is a sectional view of an in accordance with this invention;

FIG. 2 is a sectional view of an embodiment of an image amplifier tube made in accordance with this invention;

FIG. 3 is a sectional view of an image tube made in accordance with this invention; and

FIG. 4 is a partial sectional View of a pickup tube made in accordance with this invention.

Referring now to FIG. l, there is shown an image tube 10 which comprises an evacuated envelope 12 having a plurality of conducting lines 14 on an image input face 16 of the tube 10. The envelope may be made of any conventional material, e.g. glass or metal, with the Window 16 being transparent to the radiations which it is desired to detect -and amplify. The electrically conducting lines 14 may be made of any material such as, for example, evaporated -gold which has been deposited through a suitable mask (not shown) on the inner surface of the input window 16. The conducting lines 14 are spaced apa-rt to permit input radiations to pass therebetween. In this particular embodiment, the conducting lines 14 are electrically connected together and to a leadin 18. On the electrically conducting lines 14 there is provided a photoconductive material which has the property of changing its resistance in response to incident radiation. The photoconductive material 20 may be selected for its sensitivity to any desired wave Ilength of radiation, e.g. X-rays, ultra-violet, or visible, and any suitable, known photoconductive material may be used. One example of a photoconductive material which has been found suitable for the visible spectrum is a layer of activated sintered cadmium sulphide or cadmium selenide which is described in detail in U.S. Patent Number 2,765,- 385 to Thomsen issued October 2, 1956. The layer of photoconductive material 20 may have a thickness of approximately 10 microns. On the photoconductive maimage amplifying device terial 20 there is positioned a layer of magnesium oxide 22 which has the property of emitting electrons when a high electric field produces `a surface charge on the exposed surface of the magnesium oxide layer 22.

Spaced closely adjacent to the magnesium oxide layer 22 is a phosphor layer 24 which is supported by means of a transparent electrically conductive coating 26. The transparent electrically conductive coating 26 is in turn supported upon an output window 28 of the tube 10. The output window 28 is made of any material which is transparent to the radiations which are produced by the phosphor screen 24.

The phosphor layer 24 may be made of any suitable material such as willemite, cadmium sulphide or other known cathodoluminescent phosphor material. The transparent conductive coating 26 may be made of any suitable, known electrically conducting transparent material such as evaporated tin oxide.

The magnesium oxide layer 22, to function as a cold cathode, or a cold electron emitter, preferably is deposited as a porous layer on a sui-table substrate. The substrate must be a material which is capable of injecting electrons into the porous magnesium oxide layer, when under the inuence of a high applied electric field. A photoconductor, when used as such a substrate, has this desirable property of injecting electrons into the magnesium oxide layer, with the number of electrons injected into any elemental area of the magnesium oxide being controlled by the impedance of the photoconductor in that area.

The porous form of magnesium oxide may, for example, be deposited by burning magnesium in air to form a magnesium oxide smoke and then condensing this smoke on the deposited photoconductor. Alterna-tively magnesium oxide may be dispersed in an ethyl cellulose binder, and

sprayed onto the photoconductor -as a layer. The binder -is subsequently removed by heating. A layer of approximately l0 to 50 microns thick two methods described.

Other materials, for example potassium chloride, may be used as a cold electron emitter. The cold electron emitter material should be .a porous insulator with a high secondary emission ratio.

It is believed that the cold electron emitter operates in the following way. Assume that primary electrons, from some external source (not shown), land on the exposed surface of the magnesium oxide. Then, since the secondary emission ratio of the magnesium oxide is greater th-an unity, this electron bombardment provides a positive charge on the exposed surface `of the magnesium oxide. The positive charge draws or pulls other electrons from the pores of the body of the magnesium oxide layer into the space adjacent the exposed surface of the magnesium oxide where the electrons are controlled by the applied field.

The electrons that are pulled from the porous layer 22 come from the electrodes 14 through the photoconductor 20. Thus, the number of electrons that are pulled from an elemental area of the magnesium oxide layer 22 is controlled by the impedance of the photoconductor 20 beneath that elemental area.

The magnesium oxide layer 22 will function as an electron emitter, with the emission controlled by the impedance of the photoconductor, once the cold cathode has started emitting. The starting of the emission may be caused by a radiation, e.g. a light produced by a tungsten source, striking the magnesium oxide, or by some small electron source (not sh-own) adjacent to the magnesium oxide. The reason that the magnesium oxide layer 22 does not emit electrons when the high electric field, i.e. the eld between electrodes 14 and 26, is first applied, is that the cold electron emission is largely due to a high field produced as a surface charge on the exposed surface of is deposited by either of the the magnesium oxide layer. This surface charge cannot develop until some electrons leave the exposed surface of the magnesium oxide layer 22, either as secondary electrons or as photoelectrons. Incident primary electrons, to provide secondary electrons, may be provided by another electron source, e. g. an adjacent hot filament. Alternatively, the original emission may be as a result of ultra-violet radiation impinging on the magnesium oxide 'layer 22. Once the emission from the magnesium oxide layer 22 has been started, the emission can be sustained at a level of approximately amps per square centimeter, or even less. The maintained emission will appear in the form of noise from the device 10. It should be understood that the cold electron emitter, i.e. the magnesium oxide layer 22, is different from a photoemissive layer since the maintained emission continues, as long as the eld is applied between electrodes 14 and 26, even if the photoconductor is in the dark. v

During operation of the device 10, a potential dilerence is applied between all of the conducting lines 14 which are electrically connected together, and the transparent conductive coating 26. This potential difference may be of the order of 300 v. Since the photoconductor normally operates with a higher sensitivity when D.C. operation is utilized, the potential source is preferably of a D.C. type, e.g. batteries. To utilize this higher sensitivity, even though the magnesium oxide layer 22 will emit a certain number of electrons after it has once been started, the sustaining current required is adjusted, by means of a variable resistor (not shown) in the circuit, so as to be sufliciently small so that light produced by bombarding the phosphor screen 24 with the sustaining electron flow, is negligible as compared to the emission obtainable when operating at the full brightness level. For example approximately 10-3 amps per square centimeter, can be obtained when the device 10 is operating `at full brightness level. It should be noted that, in the device 10, the fluorescent or phosphor screen 24 is in close proximity to the magnesium oxide 22 so that high picture resolution is maintained. In other words, the close spacing between the cold emitter 22 and the phosphor screen 24 reduces the possibility of electron spreading between the magnesium oxide cathode 22 and the phosphor anode 24.

It should be noted that the magnesium oxide layer 22 is deposited on the photoconductor 20. Thus, the magnesium oxide layer 22 functions both as an electron emitter and as an opaque layer which prevents light feedback from the phosphor screen 24 to the photoconductor 20.

The loperation of the device 10 is such that light falling on the areas of the photoconductor 20 causes a reduction 'of impedance in each elemental area` of the photoconductor. This decreased impedance, in turn, allows more current to How from the electrodes 14, through each of `the excited elemental areas of the photoconductor 20 lthrough corresponding elemental areas of the cold cathode emitter 22 to corresponding elemental areas of the phosphor screen 24 where light is produced.

When the device 10 is in the dark, and since the resistance of the photoconductor is high, the output light of the phosphorv layer 24 is very small. However, since a `thin photoconductive layer 20 is used, it is possible to have some current flow through the photoconductor directly over each of the conducting llines 14, This current ow will show up as bright lines -on the phosphor screen 24. Thus if one desires an oscillograph type display, the conducting -lines 14 may be replaced with a conducting grid which will result in an image display on a graph-like structure.

Referring now to FIG. 2 there is shown an embodiment of this invention wherein the input window comprises a .glass surface 32 corrugated on its inner surface and supporting a plurality of electrically conducting lines 34. Each of the conducting lines 34 is positioned in the valley between two of the corrugations of the input window 32. Covering the conducting lines and lling the exposed surface of the corrugated glass 32 is a layer of photoconductive material 36. On the layer of photoconductive material 36 is a layer of magnesium oxide 33. The remainder of the tube may be the same as that described in connection with FIG. 1.

During operation of the devices shown in FIG. 2 an input image from a scene to be reproduced, excites the photoconductor which Wil-l produce a copious electron emission from the magnesium oxide layer 38 which will produce a bright visible output image. In this embodiment, no bright lines will be displayed in the light output due to the fact that the held across the photoconductive layer 36 above each of the conducting lines 34 is substantially reduced by virtue of the greater thickness or depth of the photoconductive material 36 over each of the `conducting lines. The deeper parts of the photoconductor will be more insulating due to lack of penetration by the incident light. Y

Referring now to FIG. 3, there is shown an embodiment of this invention as used in an image tube 50. The image tube 50 comprises an input window 52 and an output window 54. Positioned on the input window 52 is a light transparent electrically conducting layer 56 which may be made of a material such as tin oxide. Positioned on the transparent conducting layer 56 is a layer of photoconductive material 58. On the photoconductive `layer 58 is positioned a layer of magnesium oxide 60.

On the output window 54 there is provided a phoshor screen 62 having a light opaque, electron transparent, electrically conductive layer 64 thereon. The phosphor screen 62 may be made of any conventional materials while the light opaque, electron transparent layer 64 may be thin (eg. 1,000 Angstrom units), and made of -a material such as aluminum. Positioned around the image tube 50 is a focusing coil 65.

Thus, the embodiment shown in FIG. 3 diiers from the earlier embodiments in that a continuous light transparent electrically conductive layer is used under the photoconductor 58. Also, the output phosphor screen 62 is positioned a substantial distance from the electron emit- -ting magnesium oxide layer 60 and a focussing means, as -represented by the magnetic focussing coil 65, Vis used t0 maintain the electrons in focus in their transit through the tube.

It should be understood that the embodiment shown in FIG. 3 may use additional electrodes to provide :an electric field directly adjacent the exposed surface of the magnesium oxide, to attract the electrons from the pores of the magnesium oxide layer 60. The need for these additional electrodes will depend upon the electrode spacing `and ield produced adjacent to the magnesium oxide layer 60. A 'conventional wire mesh screen, or conventional annular Wall coating electrodes (not shown) could be utilized to produce the desired field.

Referring now to FIG. 4, there is shown an embodiment of this invention as used in an image orthicon type pickup tube. The image orthicon type tube 68 includes a photocathode in accordance with this invention. The photocathode is on an input window 70 which supports a transparent conductive coating 72. On the transparent conductive coating 72 is a layer of photoconductive material 74. On the layer of photoconductive material 74 there is provided a layer of magnesium oxide 76. Spaced from the magnesium oxide layer 76 is a conventional charge storage target 78. The charge storage tar-get 78 rmay be made of any conventional material such :as a thin roembrane of glass or a thin semi-conductive material such as magnesium oxide. The balance of the tube 68 may be a known image orthicon operated in a known manner, such as described for example, in U.S. Patent Number 2,942,132 issued to Rotow et al. or U.S. Patent 2,433,941 to Weimer.

In general, an image strikes the photoconductor which controls the electron emission from the magnesium oxide layer 76. The electronsthus emitted are accelerated onto the target 78 Where the image is stored and is subsequently read o by an electron beam to provide output signals from the tube.

Thus, applicants invention may be utilized in any type of light sensitive device and may provide an extremely thin tube such as shown in FIGS. 1 and 2. A11 advantage of this thin tube is that cathodoluminescent phosphor materials may be used which are considerably more eicient than the solid state electroluminescent materials. Assuming that the photoconductor has the same area as the magnesium oxide cathode, then the ratio of lumens input to lumens output can provide gains of 50` times or better.

I claim:

1. An image sensitive device comprising an evacuated envelope, a lig-ht transparent support within said envelope, light transparent electrode means positioned on said support, said electrode means comprising a plurality of spaceld conducting lines electrically interconnected, photoconductive material in contact with said electrode means and with said support between said lines, said material being thinner in areas of contact with said lines than in areas of contact with said support, a cold electron emitter in contact with said photoconductive material, and means spaced from said emitter for receiving electrons from said emitter.

2. An image sensitive device as in claim 1 wherein said cold electron emitter comprises porous magnesium oxide.

3. An image sensitive device as in claim 1 wherein said means for receiving electrons comprises a cathodoluminescent phosphor screen.

4. An image sensitive device comprising an evacuated envelope, said envelope including a wall portion made of light transparent material, the inner surface of said wall portion being corrugated to provide valleys, each valley being formed 'between two adjacent corrugations, conductive material in each of said valleys forming a conductive line therein, said conductive material being spaced below the plane of the peaks of said corrugations, a layer of photoconductive ymaterial over said corrugated surface and over said lines of conductive material, said layer of photoconductive material being thicker at the regions thereof over said lines of conductive material than at regions thereof over the other portions of said corrugated surface, a layer of porous magnesium oxide over said layer of photoconductive material and adapted to emit electrons, and means spaced from said layer of photoconductive material responsive in image formation to electrons emitted from said layer of magnesium oxide.

5. An image sensitive device as in claim 1 wherein said means for receiving electrons comprises a charge storage target.

References Cited by the Examiner UNITED STATES PATENTS 2,487,665 11/1949 Morton et al 313-65 2,960,617 ll/1960 Lodge et al 315-10 2,970,219 1/1961 Robert et al 313-65 X OTHER REFERENCES Thin Film Field Emission lby Malter, Physical Review, vol. 50, pages 48 through 58, Apr. 23, 1936.

HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, Examiner.

S. CHATMON, la., Assistant Examiner. 

1. AN IMAGE SENSITIVE DEVICE COMPRISING AN EVACUATED ENVELOPE, A LIGHT TRANSPARENT SUPPORT WITHIN SAID ENVELOPE, LIGHT TRANSPARENT ELECTRODE MEANS POSITIONED ON SAID SUPPORT, SAID ELECTRODE MEANS COMPRISING A PLURALITY OF SPACED CONDUCTING LINES ELECTRICALLY INTERCONNECTED, PHOTOCONDUCTIVE MATERIAL IN CONTACT WITH SAID ELECTRODE MEANS AND WITH SAID SUPPORT BETWEEN SAID LINES, SAID MATERIAL BEING THINNER IN AREAS OF CONTACT WITH SAID LINES THAN IN AREAS OF CONTACT WITH SAID SUPPORT, A COLD ELECTRON EMITTER IN CONTACT WITH SAID PHOTOCONDUCTIVE MATERIAL, AND MEANS SPACED FROM SAID EMITTER FOR RECEIVING ELECTRONS FROM SAID EMITTER. 